This invention relates generally to improvements in controlling growth processes of single crystal semiconductors for use in the manufacture of electronic components and, particularly, to a method for accurately controlling growth in a Czochralski crystal growth process for transitioning from taper growth to target diameter growth.
Monocrystalline, or single crystal, silicon is the starting material in most processes for fabricating semiconductor electronic components. Crystal pulling machines employing the Czochralski process produce the majority of single crystal silicon. Briefly described, the Czochralski process involves melting a charge of high-purity polycrystalline silicon in a quartz crucible located in a specifically designed furnace. After the heated crucible melts the silicon charge, a crystal lifting mechanism lowers a seed crystal into contact with the molten silicon. The mechanism then withdraws the seed to pull a growing crystal from the silicon melt.
After formation of a crystal neck, the growth process enlarges the diameter of the growing crystal in a cone-shaped manner by decreasing the pulling rate and/or the melt temperature until a desired diameter is reached. This portion of the crystal is typically referred to as the crown or taper. By controlling the pull rate and the melt temperature while compensating for the decreasing melt level, the main body of the crystal is grown so that it has an approximately constant diameter (i.e., it is generally cylindrical). Near the end of the growth process but before the crucible is emptied of molten silicon, the process gradually reduces the crystal diameter to form an end cone. Typically, the end cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt. During the growth process, the crucible rotates the melt in one direction and the crystal lifting mechanism rotates its pulling cable, or shaft, along with the seed and the crystal, in an opposite direction.
Although presently available Czochralski growth processes have been satisfactory for growing single crystal silicon useful in a wide variety of applications, further improvements are still desired. For example, it is desired to provide more accurate transitions from taper growth to the body target diameter.
The conventional method for transitioning from taper growth to body growth involves increasing the crystal lift rate. This causes a change in the rate of diameter increase, from some positive value to nearly zero, or even a slightly negative value. The conventional transitioning method intends to arrive at a steady-state diameter value for essentially straight crystal growth that is equal to a crystal target diameter. Currently, this transition occurs at a fixed, predetermined taper diameter regardless of the conditions inside the crystal grower. In the alternative, an operator decides when to initiate the transition. Unfortunately, the differing experience levels of various operators, in addition to differing thermal conditions inside the crystal grower, produce different taper growth rates. For this reason, the conventional methods for initiating the transition to body growth often produce differing results from one crystal growth run to another. In one instance the initial crystal body may be grown with too small of a diameter but the initial body may be grown with too large of a diameter in another. Specifically, there is often a relatively large standard deviation in crystal diameter in the early body growth compared to the desired target diameter for the crystal. This requires correction by the control system during the remainder of the body growth. Moreover, if a crystal is unacceptably undersized in its early body growth, then significant portions will be unusable for semiconductor wafer fabrication.
For these reasons, an accurate and reliable apparatus and method for controlling silicon crystal growth to automatically transition from taper growth to target diameter growth is desired.